Through an epidemiological survey, we observed 3 types of clinical courses among patients with senile dementia of the Alzheimer type (SDAT). The mental ability of the patients declined rapidly (Group A; n = 11), gradually (Group B; n = 6), or showed extremely slow changes (Group C; n = 9). The acetylcholinesterase (AChE) activity and somatostatin (SRIF) concentration of the cerebrospinal fluid (CSF) were measured in patients with Alzheimer's disease (AD) and 3 types of SDAT. Both AChE activity and SRIF concentration of CSF were significantly lower in Group A and among patients with AD compared with age-matched control subjects. Both AChE activity and SRIF concentration of CSF were not significantly different in Groups B and C. This biochemical study confirmed our epidemiological finding that only the patients in Group A with SDAT closely resembled the clinical course of AD and may belong to the category of neurodegenerative disorders.